epoll <root
EPOLL(7)                            Linux Programmer's Manual                            EPOLL(7)

NAME
       epoll - I/O event notification facility

SYNOPSIS
       #include 

DESCRIPTION
       The  epoll API performs a similar task to poll(2): monitoring multiple file descriptors to
       see if I/O is possible on any of them.  The epoll API can be used either as an  edge-trig‐
       gered  or  a  level-triggered  interface  and scales well to large numbers of watched file
       descriptors.  The following system calls are  provided  to  create  and  manage  an  epoll
       instance:

       *  epoll_create(2)  creates  an  epoll instance and returns a file descriptor referring to
          that  instance.   (The  more  recent  epoll_create1(2)  extends  the  functionality  of
          epoll_create(2).)

       *  Interest  in  particular file descriptors is then registered via epoll_ctl(2).  The set
          of file descriptors currently registered on an epoll instance is  sometimes  called  an
          epoll set.

       *  epoll_wait(2)  waits  for I/O events, blocking the calling thread if no events are cur‐
          rently available.

   Level-triggered and edge-triggered
       The epoll event distribution interface is able to behave both as edge-triggered  (ET)  and
       as  level-triggered  (LT).   The difference between the two mechanisms can be described as
       follows.  Suppose that this scenario happens:

       1. The file descriptor that represents the read side of a pipe (rfd) is registered on  the
          epoll instance.

       2. A pipe writer writes 2 kB of data on the write side of the pipe.

       3. A call to epoll_wait(2) is done that will return rfd as a ready file descriptor.

       4. The pipe reader reads 1 kB of data from rfd.

       5. A call to epoll_wait(2) is done.

       If  the rfd file descriptor has been added to the epoll interface using the EPOLLET (edge-
       triggered) flag, the call to epoll_wait(2) done in step 5 will probably hang  despite  the
       available  data still present in the file input buffer; meanwhile the remote peer might be
       expecting a response based on the data it already sent.  The reason for this is that edge-
       triggered  mode  delivers events only when changes occur on the monitored file descriptor.
       So, in step 5 the caller might end up waiting for some data that is already present inside
       the  input buffer.  In the above example, an event on rfd will be generated because of the
       write done in 2 and the event is consumed in 3.  Since the read operation done in  4  does
       not  consume  the  whole buffer data, the call to epoll_wait(2) done in step 5 might block
       indefinitely.

       An application that employs the EPOLLET flag should use nonblocking  file  descriptors  to
       avoid  having  a  blocking  read  or  write  starve  a task that is handling multiple file
       descriptors.  The suggested way to use epoll as an edge-triggered (EPOLLET)  interface  is
       as follows:

              i   with nonblocking file descriptors; and

              ii  by waiting for an event only after read(2) or write(2) return EAGAIN.

       By  contrast,  when  used as a level-triggered interface (the default, when EPOLLET is not
       specified), epoll is simply a faster poll(2), and can be used wherever the latter is  used
       since it shares the same semantics.

       Since  even  with  edge-triggered  epoll, multiple events can be generated upon receipt of
       multiple chunks of data, the caller has the option to specify the  EPOLLONESHOT  flag,  to
       tell  epoll  to  disable the associated file descriptor after the receipt of an event with
       epoll_wait(2).  When the EPOLLONESHOT flag is specified, it is the caller's responsibility
       to rearm the file descriptor using epoll_ctl(2) with EPOLL_CTL_MOD.

   Interaction with autosleep
       If  the  system  is  in autosleep mode via /sys/power/autosleep and an event happens which
       wakes the device from sleep, the device driver will keep the device awake only until  that
       event  is queued.  To keep the device awake until the event has been processed, it is nec‐
       essary to use the epoll(7) EPOLLWAKEUP flag.

       When the EPOLLWAKEUP flag is set in the events field for a struct epoll_event, the  system
       will  be  kept  awake  from the moment the event is queued, through the epoll_wait(2) call
       which returns the event until the subsequent epoll_wait(2) call.  If the event should keep
       the  system  awake  beyond that time, then a separate wake_lock should be taken before the
       second epoll_wait(2) call.

   /proc interfaces
       The following interfaces can be used to limit the amount  of  kernel  memory  consumed  by
       epoll:

       /proc/sys/fs/epoll/max_user_watches (since Linux 2.6.28)
              This specifies a limit on the total number of file descriptors that a user can reg‐
              ister across all epoll instances on the system.  The limit is  per  real  user  ID.
              Each  registered  file  descriptor  costs  roughly 90 bytes on a 32-bit kernel, and
              roughly  160  bytes  on  a  64-bit  kernel.   Currently,  the  default  value   for
              max_user_watches is 1/25 (4%) of the available low memory, divided by the registra‐
              tion cost in bytes.

   Example for suggested usage
       While the usage of epoll when employed as a level-triggered interface does have  the  same
       semantics as poll(2), the edge-triggered usage requires more clarification to avoid stalls
       in the application event loop.  In this example, listener is a nonblocking socket on which
       listen(2)  has  been  called.  The function do_use_fd() uses the new ready file descriptor
       until EAGAIN is returned by either read(2) or write(2).   An  event-driven  state  machine
       application  should, after having received EAGAIN, record its current state so that at the
       next call to do_use_fd() it will continue to read(2) or write(2)  from  where  it  stopped
       before.

           #define MAX_EVENTS 10
           struct epoll_event ev, events[MAX_EVENTS];
           int listen_sock, conn_sock, nfds, epollfd;

           /* Code to set up listening socket, 'listen_sock',
              (socket(), bind(), listen()) omitted */

           epollfd = epoll_create1(0);
           if (epollfd == -1) {
               perror("epoll_create1");
               exit(EXIT_FAILURE);
           }

           ev.events = EPOLLIN;
           ev.data.fd = listen_sock;
           if (epoll_ctl(epollfd, EPOLL_CTL_ADD, listen_sock, &ev) == -1) {
               perror("epoll_ctl: listen_sock");
               exit(EXIT_FAILURE);
           }

           for (;;) {
               nfds = epoll_wait(epollfd, events, MAX_EVENTS, -1);
               if (nfds == -1) {
                   perror("epoll_wait");
                   exit(EXIT_FAILURE);
               }

               for (n = 0; n < nfds; ++n) {
                   if (events[n].data.fd == listen_sock) {
                       conn_sock = accept(listen_sock,
                                       (struct sockaddr *) &local, &addrlen);
                       if (conn_sock == -1) {
                           perror("accept");
                           exit(EXIT_FAILURE);
                       }
                       setnonblocking(conn_sock);
                       ev.events = EPOLLIN | EPOLLET;
                       ev.data.fd = conn_sock;
                       if (epoll_ctl(epollfd, EPOLL_CTL_ADD, conn_sock,
                                   &ev) == -1) {
                           perror("epoll_ctl: conn_sock");
                           exit(EXIT_FAILURE);
                       }
                   } else {
                       do_use_fd(events[n].data.fd);
                   }
               }
           }

       When  used  as an edge-triggered interface, for performance reasons, it is possible to add
       the file  descriptor  inside  the  epoll  interface  (EPOLL_CTL_ADD)  once  by  specifying
       (EPOLLIN|EPOLLOUT).   This  allows you to avoid continuously switching between EPOLLIN and
       EPOLLOUT calling epoll_ctl(2) with EPOLL_CTL_MOD.

   Questions and answers
       Q0  What is the key used to distinguish the file descriptors registered in an epoll set?

       A0  The key is the combination of the file descriptor number and the open file description
           (also  known as an "open file handle", the kernel's internal representation of an open
           file).

       Q1  What happens if you register the same file descriptor on an epoll instance twice?

       A1  You will probably get EEXIST.  However, it is possible to  add  a  duplicate  (dup(2),
           dup2(2),  fcntl(2) F_DUPFD) descriptor to the same epoll instance.  This can be a use‐
           ful technique for filtering events, if the duplicate file descriptors  are  registered
           with different events masks.

       Q2  Can two epoll instances wait for the same file descriptor?  If so, are events reported
           to both epoll file descriptors?

       A2  Yes, and events would be reported to both.  However, careful programming may be needed
           to do this correctly.

       Q3  Is the epoll file descriptor itself poll/epoll/selectable?

       A3  Yes.   If  an epoll file descriptor has events waiting, then it will indicate as being
           readable.

       Q4  What happens if one attempts to put  an  epoll  file  descriptor  into  its  own  file
           descriptor set?

       A4  The epoll_ctl(2) call will fail (EINVAL).  However, you can add an epoll file descrip‐
           tor inside another epoll file descriptor set.

       Q5  Can I send an epoll file descriptor over a UNIX domain socket to another process?

       A5  Yes, but it does not make sense to do this, since the receiving process would not have
           copies of the file descriptors in the epoll set.

       Q6  Will  closing  a  file descriptor cause it to be removed from all epoll sets automati‐
           cally?

       A6  Yes, but be aware of the following point.  A file descriptor is a reference to an open
           file  description  (see  open(2)).   Whenever  a  descriptor is duplicated via dup(2),
           dup2(2), fcntl(2) F_DUPFD, or fork(2), a new file descriptor  referring  to  the  same
           open  file  description is created.  An open file description continues to exist until
           all file descriptors referring to it have been closed.  A file descriptor  is  removed
           from an epoll set only after all the file descriptors referring to the underlying open
           file description have been closed (or before if the descriptor is  explicitly  removed
           using  epoll_ctl(2) EPOLL_CTL_DEL).  This means that even after a file descriptor that
           is part of an epoll set has been closed, events may be reported for that file descrip‐
           tor if other file descriptors referring to the same underlying file description remain
           open.

       Q7  If more than one event occurs  between  epoll_wait(2)  calls,  are  they  combined  or
           reported separately?

       A7  They will be combined.

       Q8  Does  an  operation  on  a  file  descriptor  affect the already collected but not yet
           reported events?

       A8  You can do two operations on an existing file descriptor.  Remove would be meaningless
           for this case.  Modify will reread available I/O.

       Q9  Do  I  need  to  continuously read/write a file descriptor until EAGAIN when using the
           EPOLLET flag (edge-triggered behavior) ?

       A9  Receiving an event from epoll_wait(2) should suggest to you that such file  descriptor
           is  ready  for the requested I/O operation.  You must consider it ready until the next
           (nonblocking) read/write yields EAGAIN.  When and how you will use the file descriptor
           is entirely up to you.

           For  packet/token-oriented  files (e.g., datagram socket, terminal in canonical mode),
           the only way to detect the  end  of  the  read/write  I/O  space  is  to  continue  to
           read/write until EAGAIN.

           For  stream-oriented  files  (e.g., pipe, FIFO, stream socket), the condition that the
           read/write I/O space is exhausted can also be detected by checking the amount of  data
           read  from  / written to the target file descriptor.  For example, if you call read(2)
           by asking to read a certain amount of data and  read(2)  returns  a  lower  number  of
           bytes, you can be sure of having exhausted the read I/O space for the file descriptor.
           The same is true when writing using write(2).  (Avoid this  latter  technique  if  you
           cannot guarantee that the monitored file descriptor always refers to a stream-oriented
           file.)

   Possible pitfalls and ways to avoid them
       o Starvation (edge-triggered)

       If there is a large amount of I/O space, it is possible that by trying  to  drain  it  the
       other  files  will not get processed causing starvation.  (This problem is not specific to
       epoll.)

       The solution is to maintain a ready list and mark the file  descriptor  as  ready  in  its
       associated  data  structure, thereby allowing the application to remember which files need
       to be processed but still round robin amongst all the ready  files.   This  also  supports
       ignoring subsequent events you receive for file descriptors that are already ready.

       o If using an event cache...

       If  you  use an event cache or store all the file descriptors returned from epoll_wait(2),
       then make sure to provide a way to mark its closure dynamically (i.e., caused by a  previ‐
       ous  event's processing).  Suppose you receive 100 events from epoll_wait(2), and in event
       #47 a condition causes event #13 to be closed.  If you remove the structure  and  close(2)
       the  file descriptor for event #13, then your event cache might still say there are events
       waiting for that file descriptor causing confusion.

       One  solution  for   this   is   to   call,   during   the   processing   of   event   47,
       epoll_ctl(EPOLL_CTL_DEL)  to delete file descriptor 13 and close(2), then mark its associ‐
       ated data structure as removed and link it to a cleanup list.  If you find  another  event
       for file descriptor 13 in your batch processing, you will discover the file descriptor had
       been previously removed and there will be no confusion.

VERSIONS
       The epoll API was introduced in Linux kernel 2.5.44.  Support was added to glibc  in  ver‐
       sion 2.3.2.

CONFORMING TO
       The epoll API is Linux-specific.  Some other systems provide similar mechanisms, for exam‐
       ple, FreeBSD has kqueue, and Solaris has /dev/poll.

SEE ALSO
       epoll_create(2), epoll_create1(2), epoll_ctl(2), epoll_wait(2), poll(2), select(2)

COLOPHON
       This page is part of release 4.04 of the Linux man-pages project.  A  description  of  the
       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at http://www.kernel.org/doc/man-pages/.

Linux                                       2015-04-19                                   EPOLL(7)

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